Slobodan Praljak took the poison while appearing before United Nations judges in The Hague after his conviction and 20-year sentence were upheld.
"This has resulted in a failure of the heart, which is pointed out as the suspected cause of death".
Dutch authorities are investigating how he smuggled poison into the court.
"Praljak passed away in Westeinde hospital in The Hague after consuming a fluid substance in the courtroom of the International Criminal Tribunal for the former Yugoslavia (ICTY)", the Dutch prosecution service said in a statement.
Seconds after hearing his appeal had been lost, the former general declared: "Slobodan Praljak is not a war criminal".
HRT television showed "The Return of Katarina Kozul" in the prime 8:00 pm (1900 GMT) slot after announcing the broadcast on its website with the title "In memoriam - Slobodan Praljak".
A United Nations war crimes court said Friday it is launching an "independent expert review" into the death of a Bosnian Croat war criminal who appeared to drink poison in front of shocked judges. Everything is searched before you go in.
In a sealed letter handed over to his family a few years ago, to be opened when he dies, Praljak said he wanted that his funeral be a private one, his lawyer Nika Pinter told AFP.
"There are no guards inside the room at the time, they stay outside the door".
Though allies against the Bosnian Serbs during the 1992-95 civil war, Bosnian Croats and Muslims also fought each other for a period of 11 months, with the city of Mostar seeing some of the fiercest fighting.
The announcement came after Croatia's justice minister called into question the speed of responses by security and medical staff after Praljak, the 72-year-old former commander of Bosnian Croat military forces, shouted that he was innocent and then raised his trembling right hand to his lips and drank from a small bottle.